Two-stroke engines include an exhaust port in the cylinder wall, which is opened and closed as the piston reciprocates in the cylinder. It has been recognized that in multiple cylinder two-stroke engines reflected pressure fluctuations under idle or low speed conditions can adversely effect the combustion. At lcw or idle speeds, the pressure waves are not normally tuned so that reflected pressure waves from other cylinders in the bank, or from the same cylinder, can have an adverse effect on the engine combustion.
In view of this, it has also been proposed to utilize a butterfly valve in the exhaust port of a two-stroke motorcycle engine and control the operation of the valve through the intake manifold vacuum as disclosed in "Emission Control Of Two-Stroke Motor Cycle Engines By The Butterfly Exhaust Valve", Society Of Automotive Engineers Inc., Sept. 8-11, 1980.